Sales were effected pursuant to instructions given by the reporting person on March 9, 2012, pursuant to a Rule 10b5-1 trading plan.

The price reported in Column 4 is a weighted average price. These units were purchased in multiple transactions at prices ranging from $58.18 to $59.0401, inclusive. The reporting person undertakes to provide to MarkWest Energy Partners, L.P., any security holder of MarkWest Energy Partners, L.P., or the staff of the Securities & Exchange Commission, upon request full information regarding the number of units sold at each separate price.

Amount of securities beneficially owned following the reported transaction(s) includes Common Units owned and Restricted Units subject to varying vesting terms.